This invention relates to liquid crystal compositions comprising a nematic liquid crystal material and a specific salt and to the use thereof.
When the space between electrodes on the opposed surfaces of two base plates is filled with a nematic liquid crystal composition to form a thin transparent layer and voltage is impressed across the nematic liquid crystal layer between the electrodes with light incident on the layer, the liquid crystal layer becomes turbid where the voltage is applied, consequently blocking the light. This phenomenon is already known.
The phenomenon is utilized in light blocking systems as well as in devices for displaying characters, numerial figures, symbols, patterns, etc. It has been attempted to incorporate various additives into nematic liquid crystal materials for use in such devices to enhance the turbidity or response speed of the liquid crystal layer.
Examples of additives usually used for this purpose are quaternary ammonium halides (U.S. Pat. No. 3,656,834, No. 3,882,039), carboxylic acid salts thereof (U.S. Pat. No. 3,956,168), pyridinium halides (Japanese Patent Early Publication No. 37,883/1974) and sulfonic acid salts thereof (U.S. Pat. No. 3,963,638). However, nematic liquid crystal compositions containing these additives have the drawback that the conductivity (current) is dependent heavily on the temperature (i.e. great variations of conductivity relative to variations in temperature). In fact, when such known liquid crystal compositions are rendered equivalent to liquid crystal compositions incorporating an additive of this invention in respect of the conductivity (current) required for producing a turbidity in a low temperature range (e.g. at 0.degree. C.) with application of a.c. voltage, the former entails higher power consumption than the latter in a high temperature range (e.g. at 25.degree. C.).
For the development of turbidity, liquid crystal systems must have a conductivity higher than a definite level which is determined by the value of dielectric constant multiplied by drive frequency.
The temperature dependence of the conductivity is expressed generally by: EQU .sigma.=.sigma..sub.0 exp (-.DELTA.E/kT)
where
.sigma.: conductivity at temperature T.degree.K PA1 .sigma..sub.0 : conductivity at temperature .infin..degree.K PA1 .DELTA.e: activation energy PA1 K: Boltzmann's constant PA1 T: absolute temperature
According to the above equation, the lowest conductivity required for producing an effective turbidity in the liquid crystal system at 0.degree. C. with the application of given voltage (25 V, 60 Hz) is about 10.sup.-10.OMEGA. .multidot.cm.sup.-1. On the other hand, the conductivity .sigma. at any temperature above 0.degree. C. is such that the lower the activation energy for the conductivity, the lower will be the temperature dependence, hence savings in power consumption.
To improve the contrast ratio between the light scattering portion and the non-scattering portion, it is critical that at levels lower than the threshold voltage for producing an effective turbidity, the orientation of the liquid crystal molecules is so regulated that the long axes of the molecules are regularly positioned perpendicular to the electrode base plates. To ensure uniform perpendicular orientation, it has been common practice to apply an orienting agent to the opposed surfaces of the base plates, to add such an agent to the liquid crystal material or to treat the surfaces with an acid, but not infrequently the orienting agent applied to the base plates will dissolve out into liquid crystals, consequently varying the conductivity of the liquid crystals and degrading the crystals.
In view of the problems described above, this invention has been accomplished.